1. Field of the Invention
The present invention relates to a rubber composition, more specifically relates to a rubber composition suitable for use for a tire tread, sidewalls, or other rubber parts and having improved tensile strength, elongation, and abrasion resistance.
2. Description of the Related Art
In recent years, improvements in various types of performance have been sought in the rubber compositions for automobile and other tires etc. Therefore, in tire tread rubber and the like, several types of polymers have been used by blending together. However, when these polymers are incompatible with each other, phase separated interfaces are present. In most cases, these interfaces become starting points for breakage and are believed to have a detrimental effect on the tensile strength, tear strength, abrasion resistance, etc. However, in tire and other rubber products, since the special process of vulcanization is included, it is not possible to apply as is the molecular design of the block copolymer for control of the phase structure as is normally done in rubber/resin blends or resin/resin blends. Therefore, the problem of the phase separation interface of rubber/rubber blends has not been sufficiently studied and no method for solving this problem had been found yet.
In the past, the decrease in the breaking strength due to the incompatibility of polymer blends obtained by blending block copolymers has not been sufficiently studied. Blending, into a blend of natural rubber (NR)/polybutadiene rubber (BR), a small amount of a block copolymer of polybutadiene (BR) and polyisoprene (IR) has only been described slightly in J. Apply. Polym. Sci., 49 (1993) and RCT. 66 (1993). However, the compositions of the block copolymers used in these references have insufficient compatibility with BR, and therefore, are not satisfactory in performance for practical use. Further, attempts have been made to add cis-BR into an incompatible polymer blend of cis-BR/SBR so as to improve the abrasion resistance, but the wet braking performance is decreased, and therefore, there is a limit to the amount of addition of cis-BR and there were consequently problems in practical use. In addition, except for the proposals made by the inventors of the present invention (Japanese Unexamined Patent Publication (Kokai) Nos. 7-188510, 8-134267, 8-193147, 8-193146, 8-193145, 8-283465, 8-302071, 10-007844, and 10-036465), examples of blending a block polymer into a rubber composition as a compatibilizing agent have not been known. The previous proposals by the present inventors did not clarify the relationship between the rubber component forming the matrix of the rubber composition and the molecular weight of the block polymer added. Later study resulted in clarification of this point and the present invention has been completed.